The present invention relates to an automatic decision-point control type decision circuit suitable for use in optical receivers that automatically controls a decision point to an optimum value. The present invention also relates to a decision point controlling method.
Recently, optical amplification repeat transmission systems that amplify optical signals without regenerations and repeat them for long distance transmission are proceeding toward practical use. In such transmission systems, amplified spontaneous emission (ASE) noises which will occur in optical amplification repeaters may be induced to optical signals. Optical signal to ASE beat noises become dominant in the noise distribution in received waveforms after transmission. As a result, as shown in FIG. 5, the noise power of the mark (1) level is larger than that of the mark (0) level, so that the optimum decision point changes.
Moreover, when a change in waveform occurs because of the chromatic dispersion and non-linear effect in an optical fiber, the optimum decision phase often changes. As described above, when an optical signal is long-transmitted in an optical fiber, the value of an optimum decision point (or decision voltage, or decision phase) at which the bit error rate is minimized changes in comparison with that before transmission. This change requires readjustment of the optimum decision point. However since manual readjustment increases the cost of the system, a method of automatically controlling the decision point to an optimum value has been long waited. Conventionally, the automatic decision-point controlling method using three decision units (refer to the paper, "Smart Optical Receiver With Automatic Decision Threshold Setting and Retiming Phase Alignment" by M. Kawai, et al., in Journal of Light wave Technology, Vol. 7, No. 11 (1989), pp. 1634-1640) has been well known as an automatic decision-point adjusting method.
According to the automatic conventional decision-point adjusting method, Vth is controlled to an optimum point by obtaining an error signal based on the difference between the output of a first decision unit set to the center decision voltage Vth and the output signals of the second and third decision units set to Vth.+-..DELTA.V.
However, the conventional automatic decision-point adjusting method requires three decision units, thus resulting in the large-sized circuit configuration and increased costs. Moreover, the problem is that because branching a signal into three signals decreases the signal level, the signal to noise ratio (SNR) of a received signal is degraded.